The liver keto reductases which metabolize adriamycin and daunorubicin are heterogenous. Significant species differences in these enzymes are shown. However, the similarity of the subcellular distribution, broad specificity, cofactor dependence, and physical chemical characteristics of the aldo-keto reductases suggest these to be a class of enzymes. Microsomes contain the enzymatic machinery to catalyze anthracycline dependent electron transport. In this process the anthracyclines are reversibly converted to free radicals. BIBLIOGRAPIC REFERENCES: Bachur, N.R. and Felsted, R.L.: Oxisuran reduction by rabbit tissue preparations. Drug Metab. Disp. 4: 239-243, 1976. Bachur, N.R. and Gee, M.: Microsomal reductive glycosidase. J. Pharmacol. Exp. Therap. 197: 681-686, 1976.